Within the framework of the "DemoUpCarma"-project (http://www.demoupcarma.ethz.ch), a pilot project to study CO₂ management solution for a net-zero Switzerland, the domestic storage of CO₂ in recycled concrete was investigated. In laboratory experiments, recycled concrete aggregates (RCA) and concrete waste slurry were carbonated using 100% CO₂. In parallel, real-scale installations to carbonate both materials were implemented at a local concrete plant. In case of the RCA, different moisture saturation levels were considered, which is relevant for the real situation at the concrete plant. The carbonated materials were used to produce recycling concrete with a lower CO₂-footprint than conventional recycling concrete.

The main findings can be summarized as follows:

About 10-13 kg CO₂ per t of dry RCA (0-16 mm) can be absorbed at moisture contents of practical relevance (60-200 % of the aggregate's water absorption). Smaller size fractions (i.e. the 0-4 mm fraction) are able to absorb significantly more CO₂ than larger fractions. The carbonation leads to a patchy distribution of decalcified C-S-H on the surface of the RCA particles, which can participate in cement hydration. Concretes with carbonated RCA loose workability more rapid than those with non-carbonated RCA, but yield a higher compressive strength.

Carbonated concrete waste slurry can adsorb 120-130 kg CO₂ per t of dried material. It shows a rapid early pozzolanic reaction.

As both carbonated RCA and concrete waste slurry increase concrete performance, the cement content in the recycled concrete can be lowered. This provides an additional reduction of the CO₂ footprint besides the one due to the stored CO₂, when compared to conventional recycling concrete.

CO₂ mineralization technology using recycled concrete paste (RCP) as a sequestration substrate not only has the advantage of producing a new SCM with lower CO₂ footprint, but also complies with the circular economy concept.

The present work covers the upscaling of the enforced carbonation (EC) technology from laboratory to full scale, reporting the key learnings after the execution of industrial trial with different technologies at cement plant level.

After the laboratory development, the ideal conditions for enforced carbonation were well understood and with that, different SO₂ scrubbing technologies available in cement plants were identified to validate the laboratory findings in terms of kinetics and carbonated RCP (cRCP) performance. In the first campaign, tests were executed in a semi-dry scrubber at Brevik cement plant in Norway where the regular absorbent was exchanged for RCP, proving industrially that carbonation of RCP is feasible. The given scrubber could operate continuously in the optimal conditions. In addition to CO₂ mineralization, RCP was also found useful to sequester SO₂ and other pollutants. The analysis done in the produced cRCP allowed to estimate that 100 kg of CO₂ per ton of starting RCP were captured. This was achieved in less than 2 hours of stable operation and without a process optimization, proving that EC technology is industrially feasible. Additionally, the kiln stack analyzer showed that SO₂ emissions were reduced from 280 – 350 mg/Nm³ to 60 – 80 mg/Nm³

In the second campaign, a wet scrubber available at Ribblesdale cement plant was used for the same purpose, preparing a slurry containing RCP that replaced the standard absorbent. In this case the trial confirmed the fast kinetics of the wet carbonation process as the reaction was completed in a matter of minutes. The same final CO₂ sequestration rate as in Brevik trial was achieved. Finally, a ball mill designed for coal grinding that could be operated with kiln flue gases was successfully used to test a semi-dry carbonation of RCP to validate the concept of a reactive grinding, where continuously new surfaces are generated in the substrate to increase the process efficiency. In this case, RCP was fed to the mill instead of coal, and the process conditions were modified to maximize the retention time and relative humidity to allow carbonation to take place. In such setup, 70% of the carbonation potential was achieved, proving that this concept has a high potential to be upscaled and optimized due to its simplicity. In comparison to scrubbing technologies, a reactive grinding in a ball mill will provide the benefit of a simpler operation at lower specific energy consumption. Summarizing, the upscaling of EC of RCP has proven its promising features to be used as CCUS and allowed to identify the key aspects needed to develop and design the first-of-its-kind continuous demonstrator to be installed in a cement plant already this year.

Mineral carbonation is identified as a promising technology to reduce carbon footprint of building materials. In this study, Construction Demolition Material (CDM) consisting in pressed dewatered concrete sludge waste was used as raw material. This raw material is then carbonated following two different processes: the first at lab scale using gas with 100% CO₂ and controlled temperature / humidity conditions and the second at pilot scale. This pilot facility uses direct flue gas coming from cement plant (exhaust gas coming from chimney) for carbonation process. Whatever the operating methods, CO₂ reacts with CDM and, calcium carbonate is formed that provides a permanent CO₂ storage in this waste product. The amount of CO₂ sequestrated is related to the cement paste content in CDM and is significant: around 80 to 100 kg CO2 / t. of initial CDM. This carbonated product was then used as Supplementary Cementitious Material (SCM) at a substitution level of 25% by mass of pure CEM I. Contrary to non-carbonated sample that behaves as limestone filler regarding its reactivity in cement, a higher reactivity of the carbonated sample is measured with a pozzolanic / activity index close to fly ash one . A subsequent thermal treatment at medium range around 450°C of the carbonated sample enables to reach even higher activity index between fly ash and Granulated Ground Blast Furnace Slag. These promising results highlight that carbonated CDM followed or not by thermal activation can be used as alternative SCM in cement without compromise in mechanical performance. Carbonation processes enable the valorization of recycled CDM as SCM in cement and with additional contribution to the reduction of carbon footprint of the binder by permanent CO₂ storage.